Electric heating unit



p 1935- c. c. ABBOTT 2,036,788

ELECTRIC HEATING UNIT Filed March 6, 1934 Tnvent'ov: Chcwies C. Abbott,

l His AttOTTNQg.

Patented Apr. 7, 1936 UNITED STATES PATENT OFFICE ELECTRIQ HEATING UNITNew York Application March 6, 1934, Serial No. 314,286

3 Claims.

This invention relates to electric heating units, more particularly toelectric heating units of the sheathed type, and it has for its objectthe provision of an improved heating unit of this 5 character.

This invention has particular application to sheathed heating units ofthe type described and claimed in my United States Patent No. 1,367,341dated February 1, 1921. As there described, this type of heating unitcomprises a resistance conductor encased by a metallic sheath andembedded in and supported in spaced relation with respect to the walls01 the sheath by a powdered heat refractory electrically insulatingmaterial,

5 such as magnesium oxide, compacted to a hard dense mass by reducingand elongating the sheath, as by rolling or swaging.

The metallic sheath of this heater is formed of a metal which is capableof being materially as reduced and elongated, such as steel, in orderthat the insulating material can be compacted into a mass suflicientlydense to readily conduct heat from the resistance conductor to thesheath.

Q5 In certain applications, it has been found that sheaths formed ofsteel and other like commercially practical metals capable of beingWorked sufiiciently to compact the insulating material will corrode ordeteriorate and after a time fail so completely. For example, it hasbeen found that heaters having steel sheaths corrode and ultimately failwhen used to heat water in certain applications, such as in the coolingsystems of mercury arc rectifiers.

35 In this application, it has been discovered that pure nickel will notcorrode or deteriorate asdoes the steel sheath, but it has been verydifficult and practically impossible in certain cases to make anelectric heating unit with a 40 pure nickel sheath because nickel cannotbe worked mechanically so as to compact the insulating materialsufficient-ly. When subjected to the necessary reducing operations, thenickel splits up and cracks open, thereby ruining the 45 heater.

In accordance with this invention, the heating unit is made with asheath formed of steel, or some other mechanically strong metal capableof being worked sufiiciently to compact the in- 50 sulating material.After the sheath has been assembled with the resistance conductor andits insulating material and the sheath has been reduced and elongated inorder to compact the insulating material, the sheath is slipped into 55or encased by a second sheath formed of nickel.

(Cl. Mil-6'3) This second sheath is then reduced and elongated justenough to cause it to grip tightly the inner sheath so as to effect agood thermal and mechanical connection between the two sheaths.

For a more complete understanding of this invention, reference should behad to the accompanying drawing in which Fig. l is an elevation partlyin section of an electric heating unit embodying this invention; andFig. 2 is a secit tional view taken through the line 2-2 of Fig. l andlooking in the direction of the arrows.

Referring to the drawing, this invention has been shown as applied to asheathed heating unit of the type described and claimed in my ill abovement'oned Patent No. 1,367,341 intended to be used to heat water in thecooling system of a mercury arc rectifier. Briefly, this heatercomprises a helical resistance conductor l6 encased by a metallic sheatit formed of any suitable material, such as steel. The helicalresistance element ill is embedded in and supported in spaced relationwith respect to the sheath ll by means of suitable heat refractoryelectrically insulating material l2, such as powdered magnesium oxide.The powdered magnesium oxide i2 is compacted to a hard dense mass so asto readily conduct heat from the resistance element ill to the sheath Mby reducing and elongating the metallic sheath ll. This may beaccomplished by swaging or rolling. The ends of the sinuous resistanceelement iii are electrically and mechanically connected to suitableterminal members l3, the left-hand terminal only being shown.

As pointed out previously, the steel metallic sheath M will deteriorateand corrode in certain water heating applications, such as in coolingsystems for mercury arc rectifiers and the like. The heater has acomparatively short life when used in such applications.

In accordance with this invention, the sheath ii is covered by a secondsheath H3, which is formed of any suitable metal which will notdisintegrate or corrode in the presence of moisture and water, such assubstantially pure nickel. Preferably, and as shown in Fig. 1, thesheath l4 will have substantially the same length as has the sheath ll.

The steel sheath ll is assembled with the nickel sheath i4 after thesheath H has been assembled with its heating unit and the magnesiumoxide l2, and the latter has been compacted. Thus, in making the heaterthus far described, the resistance conductor 10 is me- 5 chanically andelectrically connected with its terminals II. This may be doneconveniently in the manner described and claimed in my Patent No.1,494,938, dated May 20, 1924. As there described, each terminal I3 isprovided with a threaded portion l5 upon which the associated end turnsof the helical resistance conductor are turned or screwed. The diameterof the helix is somewhat less than the diameter of the terminal at theroots of the thread so that the helix is extended somewhat when turnedon. When the conductor subsequently contracts by reason 01' itsresiliency on the threaded portion of the terminal, it will make a verygood electrical and mechanical connection with the terminal.

Alter the conductor has been assembled with its terminals, it isthreaded into the sheath II and is supported in a central positionwithin the sheath by its terminals which are supported in any suitablemanner, not illustrated. After this, the sheath II is loaded with themagnesium oxide l2 and then is reduced and elongated so as to firmlycompact the oxide within the sheath, as described in my above-mentionedPatent No. 1,367,341. As pointed out previously, the sheath ll may bereduced and elongated in order to compact the oxide I! by rolling orswaging.

After this operation, the sheath II is fitted into or encased by thesheatlr I4. Preferably, the sheath l4 will have an internal diameterwhich will just permit the sheath II to he slid easily into it after thesheath II has been reduced and elongated to compact the oxide. Then thenickel sheath I4 is reduced and elongated just enough to cause it totightly grip the inner sheath II. In a specific example,- the sheath Hhas an original internal diameter Just sufficient to permit the sheathII to be slid into it, and then its diameter is reduced by approximately.010 or .015 inch. This reduction will be sufiicient to cause the outersheath H to grip the inner sheath ll sufiiciently tightly to effect agood thermal and mechanical connection between these members.

Arranged within the end portions of the inner sheath II are masses ofelectrically insulating sealing material l6, arranged to seal the heateragainst the pusage of fluids, such as moisture and gases, through theends of the heater. While any suitable sealing material l6 may be used,it is preferable to use glass having a high electrical resistance at theoperating temperature of the heater. In making the seals, glass tubeshaving substantially the diameter of inner sheath ll, after it has beenreduced, will be threaded upon the terminals l3, as shown in Fig. 1,then will be heated substantially to the plastic state, and then whilepressure is applied allowed to cool. The glass l6 thus treated will becaused to adhere to the terminals I3 and to the surrounding inner wallsof the sheath H with fluid impervious joints.

After the seals have thus been formedat the ends of the heater, suitableelectrically insulating washers I! are threaded on the terminals so asto cover the .sealing material It and the end walls of the sheaths H andI4. These washbut I prefer to anchor them to the terminals by providingnicks in the opposite sides of the terminals into which the metal of theconnectors is forced or compressed. It will be understood that themembers I8 will be formed of a suitable relatively soft material, suchas brass, so that it will lend itself to the flattening or compressingoperation. This method of securing the connectors to the terminals isdescribed and claimed in my above-mentioned Patent No. 1,494,938.

Each lug or connector 18 is provided with a threaded portion "aupon'which are threaded several nuts 2|. These nuts serve to securesuitable leads 22 to the connectors. Preferably, cupshaped washers 23will be interposed between the inner nuts 2| and the leads 22, asclearly shown in Fig. 1.

While I have shown a particular embodiment of my invention, it will beunderstood, of course. that I do not wish to be limited thereto sincemany modifications may be made, and I, therefore, contemplate by theappended claims to cover any such modifications as fall within the truespirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. An electric heater comprising a metallic sheath capable of beingmaterially reduced and elongated, a resistance element within saidsheath, heat refractory electrically insulating material within saidsheath embedding said resistance element and compacted to a hard densemass by elongating and reducing said sheath, a second metallic sheathformed mainly of nickel covering said first named sheath and fittedtightly thereto in good thermal relation with it s by reducing andelongating said second sheath on said first sheath.

2. An electric heating unit comprising a steel sheath, powdered heatrefractory electrically insulating material within said sheath compactedto a hard dense mass by reducing and elongating the sheath, a resistanceconductor within said sheath embedded in said mass out of contact withsaid sheath, terminals connected to said resistance conductor projectingfrom said steel sheath, a second sheath formed substantially of purenickel encasing said steel sheath and elongated and reduced in diametersufilciently to give it a good mechanical and thermal relation with saidsteel sheath, and masses of electrically insulating sealing materialwithin the end portions of said steel sheath surrounding said terminalsand having fluid impervious joints with said terminals and thesurrounding walls of said sheath.

3. An electric heater comprising a metallic sheath capable of beingmaterially reduced and elongated, a resistance element within saidsheath, heat refractory electrically insulating materialwithin saidsheath embedding said resistance element and compacted to a hard densemass by reducing and elongating said sheath, and a second sheath formedof a selected material, which is incapable of being worked mechanicallyto reduce and elongate it sufficiently to compact said insulatingmaterial, covering said first sheath and reduced and elongatedsufficiently to cause it to firmly grip said first sheath, whereby agood mechanical and thermal relation is established between saidsheaths.

CHARLES C. ABBO'I'I.

